How to make ag search "non-standard" langauge files?

I'm using Silver Searcher, ag, version 2.1.0 on Ubuntu. I'm trying to search for the string aStar in Haskell files in the current directory tree. (The files are text files with extension .hs.) The behaviour isn't what I expect.

$ ag aStar

finds no matches.

ag --haskell aStar

finds no matches.

ag --haskell --all-text aStar

finds the correct matches in the .hs files.

ag --all-text aStar

finds the correct matches in .hs files, plus a bunch in some temporary files I don't want searched. Any suggestions on what I'm doing wrong, and why the --haskell flag doesn't make ag search the Haskell files?